Many commercial processes, particularly in the electronics industry, require high purity gases and gas mixtures, i.e., gases and gas mixtures which are substantially free from one or more unwanted components (hereinafter referred to as "impurities"). In many applications the high purity gases and gas mixtures must contain less than a total of 50 parts per million (ppm) nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrocarbons and water. In other applications the only requirement may be that the high purity gas mixture contains less than a few parts per million of a specific impurity, for example water. In these cases the presence of small quantities of other gases may be unobjectionable.
More recently even more stringent specifications have been set down requiring less than several parts per billion (10.sup.9) impurities.
In order to provide such high purity gases it is conventional to store the gases (or the liquefied gas) in specially prepared cylinders, the inner surfaces of which have been highly polished and which are typically purged, evacuated and baked before use. However, even with such care it is still necessary to provide a purifier immediately upstream of the point of use. This purifier will typically operate at about 2 bar gauge.
Existing purifiers contain a selection of proprietary materials and have to be returned to the manufacturer for the old material to be removed and new material to be loaded at frequent intervals. The proprietary material is not sold separately and obtaining a recharged unit is very expensive. Furthermore, the purifier itself has to be manufactured to very high standards to prevent the ingress of impurities from the atmosphere and is thus itself very expensive.
It will be appreciated that the overall cost of operations could be decreased by ensuring that the gas leaving the cylinder is as pure as practicable, thereby reducing the duty on the purifier or potentially obviating the need for a purifier altogether.
U.S. Pat. No. 4,032,311 discloses the concept of providing a filter in a compressed air tank for use in diving. The filter is mounted on the inlet of the control valve on the tank and prevents flakes of rust from the tank or particulates introduced by the compressor during filling from entering the diver's pressure regulator. This prior art is not concerned with the supply of high purity fluid.